11 Dec 2018

Deep-bore injection is unlikely to be a suitable method for disposing of Akaroa’s treated wastewater, test drilling has shown.

Christchurch City Council has completed drilling at two sites on the outskirts of Akaroa. The first hole, on Old French Road, was drilled to a depth of 250 metres.

Kylie Hills with some of the core samples.

Senior Project Manager Kylie Hills with some of the core samples taken from the outskirts of Akaroa.

The second hole, at the site of the new wastewater treatment plant at the top of Old Coach Road, was drilled to a depth of 150 metres.

Senior project manager Kylie Hills said the drilling showed the rock to be highly competent (lightly fractured) basalt and tuff (a mix of rock and volcanic ash).

“We wanted to know if the area around Akaroa was suitable for deep-bore injection,” Mr Hills said. “We found some fracturing, but the fractures across the target depths were high in mineral deposits, which had infilled and restricted many of the flow paths through the rock.”

Mr Hills said it would take many more holes than expected to achieve the flow rate needed to dispose of the town’s wastewater, making it much more expensive than other options available.

Test drilling and site remediation has cost $550,000. At this stage, there are no plans to do more drilling.

Mr Hills said the core recovered from the first hole is now on loan to the University of Canterbury’s geology department.

“They’re pretty interested in it because it’s the only drill core ever recovered at these depths from the Akaroa area,” he said. “It offers a unique insight into the volcanic history of Banks Peninsula.”